For this study we exposed four rhesus macaque fetuses at 75 days gestation to infectious retroviral vector to evaluate in utero gene transfer as a means of developing rhesus macaques for evaluating gene therapy approaches for AIDS. We had previously shown that the fetal liver of rhesus macaques at 65-75 days gestation contains cell populations having hematopoietic stem cell (HSC)-like qualities, and that the fetal liver expresses the Gibbon ape leukemia virus receptor (glvr) at high levels. With these results we hypothesized that retroviral-mediated gene transfer (RMGT) may be more efficient in developing fetuses, as HSCs are in the microenvironment that promotes cell proliferation - a condition required for RMGT with murine retrovirus-based vectors. The fetuses were separated into two groups, two fetuses per group. Each group 1 fetus was infected with 1.0 mL of culture supernatant from the PG13/LNc8 cell line, which produces infectious retroviral vector at titers of 1 x 106 colony forming units (CFU). One fetus was given the control retroviral vector, LN, while the second fetus was given the retroviral vector containing the trans-dominant negative HIV-1 revA5. Each group 2 fetus was infected with 1.0 mL of culture supernatant containing concentrated retroviral vector pseudotyped with the VSV envelope protein. The first fetus was given infectious retroviral vector having a titer of 1 x 108 CFU, while the second fetus was given infectious retroviral vector having a titer of 2 x 107 CFU. Both group 1 fetuses survived to term and were delivered by C-section 5 days prior to parturition for isolation of cord blood for analysis. The first group 2 fetus died in utero and the second group 2 fetus survived to term. The fetuses that survived have since grown, appear clinically normal and are being evaluated for evidence of stable gene transfer in cell types derived from HSCs.